To increase the capacity of a data transmission, a transmission system that generates and/or recovers a signal by using a digital signal processing technique has been put into practical use in recent years. For example, in a backbone transmission system, a transmission system that provides 100 Gbps by using polarization-multiplexed multilevel modulation and digital coherent reception is in practical use.
FIG. 1 illustrates an example of an optical transceiver. The optical transceiver 1000 includes a digital signal processor (DSP) 1001, a D/A converter (DAC: Digital-to-Analog Converter) 1002, an optical transmitting device (Tx) 1003, an optical receiving device (Rx) 1004, and an A/D converter (ADC: Analog-to-Digital Converter) 1005 as illustrated in FIG. 1.
The DSP 1001 generates a transmission signal from an input data signal with digital signal processing. The D/A converter 1002 converts the transmission signal generated by the DSP 1001 into an analog signal. The optical transmitting device 1003 generates an optical signal based on an output signal of the D/A converter 1002. The optical receiving device 1004 generates a received signal that represents a received optical signal. The A/D converter 1005 converts the received signal into a digital signal. Then, the DSP 1001 recovers a data signal by demodulating an output signal of the A/D converter 1005 with digital signal processing.
The DSP 1001, the D/A converter 1002 and the A/D converter 1005 are mounted, for example, on one IC chip. In FIG. 1, the DSP 1001, the D/A converter 1002 and the A/D converter 1005 are mounted on a DSP-DAC/ADC chip 1006. Moreover, the DSP-DAC/ADC chip 1006, the optical transmitting device 1003 and the optical receiving device 1004 are mounted, for example, within one module. In FIG. 1, the DSP-DAC/ADC chip 1006, the optical transmitting device 1003 and the optical receiving device 1004 are mounted within an optical transceiver module 1007. In this case, the optical transceiver module 1007 is implemented, for example, on a motherboard.
Related techniques are described, for example, in International Publication Pamphlet No. WO97/14093 and Japanese laid-open Patent Publication No. 2010-63173.
In optical transmission equipment including an optical transceiver, it is sometimes demanded that optical devices (the optical transmitting device 1003 and the optical receiving device 1004 in FIG. 1) are removable. In the following description, a configuration in which a device is removable from the optical transmission equipment is sometimes referred to as being “pluggable”.
Here, to realize a large-capacity optical transmission equipment, it is preferable that a size of a pluggable module is small in order to increase a mounting density of an optical transceiver. Accordingly, standards such as CFP2, CFP4 and the like are proposed to downsize a pluggable module.
However, in the optical transceiver 1000 illustrated in FIG. 1, the DSP-DAC/ADC chip 1006 including the DSP 1001 consumes high power. Moreover, it is difficult to downsize the DSP-DAC/ADC chip 1006 in comparison with optical devices. For this reason, when a pluggable module including optical devices is downsized, it is difficult to implement the DSP-DAC/ADC chip 1006 within the pluggable module. Accordingly, in this case, the pluggable module 1008 and the DSP-DAC/ADC chip 1006 are separated and arranged on a motherboard as illustrated in FIG. 2. Namely, a signal line between the pluggable module 1008 and the DSP-DAC/ADC chip 1006 becomes long.
In the configuration illustrated in FIG. 2, an analog signal is transmitted between the pluggable module 1008 and the DSP-DAC/ADC chip 1006. When a bit rate or a symbol rate of a data signal is high, also a speed or a frequency of the analog signal transmitted between the pluggable module 1008 and the DSP-DAC/ADC chip 1006 becomes high. Moreover, it is difficult to shorten the signal line between the pluggable module 1008 and the DSP-DAC/ADC chip 1006 as described above. Accordingly, there is a probability that a waveform of the signal is degraded between the pluggable module 1008 and the DSP-DAC/ADC chip 1006. When the waveform of the analog signal is degraded, also transmission characteristics such as a bit error rate and the like are deteriorated.
The above described problems do not occur only in a case where optical devices are implemented within a pluggable module. Namely, in an optical transceiver using digital signal processing, transmission characteristics are sometimes degraded when the degree of freedom of an arrangement of optical devices and a DSP is improved. Moreover, the above described problems do not occur only in an optical transceiver including an optical transmitter and an optical receiver. Namely, the above described problems may also occur in an optical transmitter using digital signal processing, or an optical receiver using digital signal processing.